This invention presents a novel treating process for the removal of trace quantities of olefinic impurities from hydrocarbon process streams containing predominantly cyclic compounds, specifically aromatic and naphthenic hydrocarbons. More particularly, this treating process uses a solid medium comprising crystalline aluminosilicate zeolite and refractory inorganic oxide to reduce the olefin level of hydrocarbon process streams to less than 50 Bromine Index. A more efficient olefin removal process with minimal deleterious transalkylation reactions is achieved with the present invention compared to processes of the prior art.
Olefinic hydrocarbon impurities are commonly found in aromatic hydrocarbon process streams originating from naphtha fractions of petroleum and from reforming or steam cracking of naphthas. Alternatively, olefins can also be formed as by-products in a number of petrochemical processes, for example, in isomerization and transalkylation processes. Removal of these olefins is important for several reasons. One is to meet product specification or purity standards required by aromatic producers. If the olefins are not removed, there is a high probability that they will undergo reaction with other compounds to produce undesirable coproducts, such as, colored materials in benzene sulfonates. Olefin removal is also important in order to protect certain petrochemical processes which are particularly sensitive to olefins even in concentrations as low as a few parts per million. For example, in a separation process that uses molecular sieves, olefins will occupy sieve capacity and thereby adversely affect separation performance. Another reason for removal is because olefins can cause fouling in high temperature equipment, such as, in a xylene column reboiler. The current practice for removing olefin contaminants involves the use of the clay treating process. Here a hydrocarbon stream is contacted with a clay, such as, activated bentonite, Attapulgus clay, fuller's earth, Superfiltrol, Floridin, and the like. These clays are composed primarily of amorphous and crystalline mixtures of silica and alumina. However, although some types of clays exhibit a minor degree of crystallinity, they are readily distinguishable from the structure and crystallinity of crystalline aluminosilicate zeolites. The acidic nature of the clays cause the olefins to react with the aromatics present via an alkylation reaction whereby heavy hydrocarbons are produced which are subsequently removed by fractional distillation.
An example of such a clay treating process is taught in U.S. Pat. No. 2,778,863 (Maisel et al). This reference teaches a process to remove olefins from C.sub.6 and C.sub.7 aromatic hydrocarbon concentrates by successive contacting with clay followed by intermediate fractional distillation. The clays of the U.S. Pat. No. 2,778,863 reference are of the naturally occurring type. Similarly, U.S. Pat. No. 3,835,037 (Fairweather et al) teaches the removal of color-generating olefinic impurities from aromatic hydrocarbons via oligomerization/polymerization at low temperature using either a natural occurring clay material or a sythetic alumina and/or silica-containing material, e.g., silica-magnesia, silica-zirconia, etc.
Another approach to the problem of removing impurities from aromatic containing hydrocarbon streams is described in U.S. Pat. No. 3,400,169 (Eng et al). Here, a slurry of silica alumina catalytic cracking catalyst and steam cracked naphtha is contacted with a second stream of steam cracked naphtha in a tower zone to polymerize olefins and convert thiophenes to paraffins and H.sub.2 S. The catalyst of this process may contain 13-30% of alumina and 70-87% silica.
We have now found a treating process for removing olefinic impurities from process streams containing substantially aromatic hydrocarbons which does not use the traditional amorphous clay material, but instead employs a crystalline aluminosilicate zeolite and a refractory inorganic oxide. Treating of such process streams also does not significantly change the quantity or distribution of the aromatic hydrocarbons treated.